This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This study will test an experimental medication, Phenoptin, in subjects who have hyperphenylalaninemia due to a primary BH4 deficiency. Hyperphenylalaninemia (HPA) is a very rare disease, estimated to have 450 sufferers worldwide. There is no cure. The mechanism to control this disease is done through strict diet control in order to maintain a low blood phenylalanine level.